There are many situations where circuitry has to analyze and evaluate various pulses in a signal train. A part of this processing may include attempting to synchronize with a known but apparently randomly spaced set of signal pulses (pseudorandom). When the pulse train includes additional repetitiously occurring signal pulses, the detection circuitry can become overloaded with signals to process and accordingly, is sometimes unable to properly perform its intended detection or synchronization function.
It is accordingly desirable, depending upon the frequency of the potentially interfering signals to provide a low-pass, a high-pass, or possibly a bandpass filter for eliminating the repetitiously occurring signals from the train of signal pulses prior to the train of pulses being presented to the detection circuitry. The present invention illustrates in detail a filter which may be designated as a low-pass filter in that repetitiously occurring signal pulses of a low frequency are passed while high frequency repetitiously occurring pulses are deleted and non-repetitiously occurring signals are only randomly deleted, when they happen to occur at substantially the same time as a repetitiously occurring signal, with the random deletions being few enough so as to not substantially affect the performance of the detection circuitry downstream.
The present invention accomplishes the deletion of the repetitiously occurring signals by placing one or more circuits of the present design in series between the signal source and the synchronization circuitry downstream to be protected. Each circuit counts the time between a first pulse and a subsequent pulse. The time between the first pulse and the subsequent pulse is placed in a down counter. When the down counter reaches substantially zero, a blanking signal is applied to the pulse train to delete any signals from passing to the downstream circuitry. If a pulse is detected coincident with the blanking signal, it is assumed that a repetitious signal has been detected. The down counter is then again loaded with the previously loaded count for as long as a pulse is detected coincident with the blanking signal. When a pulse is no longer detected as being received during (coincident with) the blanking signal, the circuit starts a new measurement and blanking cycle. If, on the first up count timing attempt, a pulse is not detected subsequent to the further signal, and if a predetermined maximum time has not been exceeded, the circuit checks for any further received signal pulses, and if any signal is detected, this new count is supplied to the down counter, and a check is made to see if a further repetitiously occurring signal is received coincident with the time that the down counter again approaches a zero count. The feature of returning to the original up count if a signal is not detected immediately subsequent to the initial pulse and first subsequent pulse helps eliminate confusion to the filter in situations where there are two interfering repetitiously occurring signals where the phase of one is slightly offset from the other. The first filter in a tandem set of two can eliminate the first repetitiously occurring signal, and the second filter can shortly thereafter eliminate the second repetitiously occurring signal. This leaves the remaining asynchronous signal pulses to be applied to downstream detection circuitry.
It is therefore an object of the present invention to provide a repetitiously occurring pulse rate reject or filter circuit.